High resolution scanning tunneling spectroscopy of ultrathin iron silicide grown on Si(111): Origin of the c ( 4 × 8 ) long range order

Thin iron silicide grown on Si(111) undergoes a $p(2\ifmmode\times\else\texttimes\fi{}2)\ensuremath{\rightarrow}c(4\ifmmode\times\else\texttimes\fi{}8)$ ordering process upon annealing that was studied by scanning tunneling spectroscopy. We identify unambiguously 3 nonequivalent Si adatom sites in the $c(4\ifmmode\times\else\texttimes\fi{}8)$ unit cell and a well localized electronic state that exhibits $p1m$ spatial group symmetry as opposed to the 2 sites and $p2mm$ symmetry revealed so far by standard topographs. Amazingly this electronic state originates in an atomic defect in the fifth subsurface layer, right at silicide-substrate interface and the $c(4\ifmmode\times\else\texttimes\fi{}8)$ periodicity reflects long range ordering of these defects.